Up to now, carbon black-based pigments and iron oxide-based pigments have been general as black pigments that are used as coloring agents for paints, printing inks, and plastics. These black pigments exhibit a black color by absorbing all the rays of light including visible light region of sunlight.
The black pigments (carbon black-based pigments in particular) absorb light in a visible light region (about 380 to about 780 nm) to exhibit a black color, but in fact, also absorb light that is in a near-infrared region including a wavelength region of 800 to 1,400 nm and that greatly contributes to heat. Therefore, there has been a problem that the temperature of articles colored by the black pigments as described above rises easily by being irradiated with sunlight. In addition, as articles colored by a black pigment, sophisticated products such as a black matrix for a color filter have appeared in recent years.
Electric insulation is demanded of a black pigment to be used for constituting a black matrix for a color filter in order to prevent malfunction of a thin film transistor. Carbon black is a pigment having a low electric resistance. Therefore, it cannot be said that carbon black is not appropriate as a coloring material to be used in products such as a black matrix, and it is desirable to use a light-shielding material having a more excellent electric insulation. As related conventional techniques, various electrically insulating black organic pigments whose temperature is unlikely to rise even if they are irradiated with sunlight have been studied (Patent Literature 1). It is to be noted that isoindolinone compounds each having a particular structure, the isoindolinone compounds each being useful as a pigment for coloring a polymer organic material and the like, are proposed (Patent Literatures 2 and 3).
Examples of other uses of a black organic pigment which has attracted attention in recent years include a coloring agent to be used for forming a plastic product for use in laser resin welding. The laser resin welding is a method of welding plastic products without using an adhesive by combining a laser-transmissible resin layer and a laser-absorptive resin layer and performing irradiation with laser of wavelength in a near infrared region (for example, 800 to 1100 nm). Various black organic pigments which are useful as a coloring agent for forming a plastic product for use in laser resin welding have so far been studied.